Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline Sunday. Avoid slopes steeper than 30 degrees or terrain where avalanches could release from above.

Issued: 8:10 PM PST Saturday, February 17, 2018

by Dennis D'Amico

Elevation

Sunday

Outlook

The Day 2 avalanche danger outlook is different than the Day 1 avalanche danger
forecast. The Day 2 avalanche danger outlook has a higher level of uncertainty in
the regional avalanche danger but can still show the expected danger level trend
versus the Day 1 forecast. Also, the Day 2 avalanche outlook does not have avalanche
problems defined as a result of the increased uncertainty.

Outlook for Mondayi

Above Treelinei

Above Treeline

Above Treeline is the highest of the three elevation bands used in NWAC
forecasts. It includes alpine areas where the most snow falls and the winds
are the strongest. Above Treeline is not an exact elevation, but a
characteristic of the terrain, beginning as the treeline transitions into
open slopes extending to ridges and to the tops of the ridges or
non-volcanic peaks and to the cascade crest level on the Cascade volcanoes.
Due to the lack of high alpine snowpack data available to the forecasters,
the upper limit roughly corresponds to a 7-8,000 level.

An avalanche that started above treeline and ran to lower elevations.

Sunday

High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Outlook for Mondayi

Considerable

Near Treelinei

Near Treeline

Near Treeline is the middle of the three elevation bands used in NWAC
forecasts. It is a transition zone between dense forests and treeless alpine
areas. It is the narrowest of the three elevation bands, extending only a
few hundred feet above and below the treeline. It varies locally, and is not
a constant elevation or width.

An avalanche that started and ran near treeline.

Sunday

High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Outlook for Mondayi

Considerable

Below Treelinei

Below Treeline

Below Treeline is the lowest of three elevation bands used in NWAC forecasts.
It extends from valley floors, or snowline, to Near Treeline. Open areas and
sparse trees are possible. Snowfall tends to be less than the other
elevation bands. Tree cover shelters the snow from wind effects.

Outlook for Mondayi

Moderate

Danger Scalei

Danger Scale

The North American Avalanche Danger Scale is a five level scale used to communicate the
danger of avalanche to backcountry recreationalists. The danger is a combination of the
expected likelihood, size, and distribution of avalanches. It provides a very basic
description of the avalanche conditions.

No Rating

Low

Moderate

Considerable

High

Extreme

Issued: 8:10 PM PST Saturday, February 17, 2018

by Dennis D'Amico

Avalanche Problems for Sunday

Avalanche Problems

Avalanche Problems are categories of avalanche activity. The Problems may not describe
all avalanche activity you might observe, but they categorize the avalanches by how we
manage the risk in the terrain. This approach focuses on relevant observations you can
make in the field and how to treat the avalanche risk.

The forecasts list up to three current Problems, along with the spatial distribution, the
likelihood of avalanches, and anticipated size. Forecasters may provide specific details
to a Problem in the forecast discussion tab.

This link has detailed descriptions of
each Avalanche Problem and suggestions for risk treatment.

Wind Slabi

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Wind Slab

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

Avalanche Problemi

Aspect/Elevationi

Likelihoodi

Sizei

Storm Slabsi

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Avalanche Problemi

Aspect/Elevationi

Likelihoodi

Sizei

Aspect/Elevation

The Aspect/Elevation diagram describes the spatial pattern of the Avalanche Problem by aspect (the
direction a slope faces) and elevation band (Above, Near, or Below Treeline). The diagram will be
filled with black where the Avalanche Problem may exist. You can view the diagram as you would a
mountain on a topographic map. The outer ring represents the Below Treeline elevation band, middle
ring Near Treeline, and the inner ring Above Treeline. The diagram is oriented like a compass, with
the top wedges representing north aspects, the left wedges representing west, etc.

Likelihood

Likelihood is a description of the chance of encountering a particular Avalanche Problem. It combines
the spatial distribution of the Problem and the sensitivity or ease of triggering an avalanche. The
spatial distribution indicates how likely you are to encounter the Problem in the highlighted
avalanche terrain. The sensitivity indicates how easy it is to trigger avalanches including both
natural and human triggered avalanches.

Size

Size is based on the destructive potential of avalanches.

SMALL avalanches are relatively harmless to people unless they push you into a terrain trap.

LARGE avalanches could bury, injure or kill a person.

VERY LARGE avalanches could bury cars, destroy a house, or break trees.

HISTORIC avalanches are even more destructive, and nearing the maximum size the slope could
produce.

Recent images from NWAC:

Avalanche Summary:

A strong storm system brought heavy precipitation in the form of snow, ice and rain to Mt. Hood Saturday along with moderate winds increasing to strong and gusty winds in the afternoon (WSW gusts 40-70 mph were common at the lower NWAC wind sites!). Scoured surfaces surfaces are expected on windward slopes as well as wind effected snow well into the below treeline band. Despite heavy precipitation, only about 4 inches of new snow was recorded at the Meadows and Timberline base plots through 5 pm Saturday. The rain-line is estimated to have reached near 5500 feet Saturday mid-day before cooling ensued.

Prior to this system, 12-18 inches of storm snow fell late last week and sits on a strong firm crust layer formed earlier in the month. There are no significant layers of concern in the mid and lower snowpack.

Observations

On Saturday morning, a new rain or freezing rain crust was noted up to 6600 ft. Rime ice had covered most surfaces. Strong winds prevented observations later in the day.

Forecast for Sunday:

Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline. Avoid slopes connected to large start zones higher in the terrain where wind slab avalanches are capable of producing very large and destructive slides.

A strong cooling trend should help mitigate the storm slab likelihood, but at the same time preserve storm slab instabilities formed overnight. In non-wind affected terrain, avoid open slopes greater than 35 degrees and allow new storm slab instabilities time to stabilize.

New snow will likely bond poorly to a slick crust that formed Saturday morning near and below treeline. This would be a likely bed surface for any avalanche that steps down to deeper layers.

Avalanche Summary:

A strong storm system brought heavy precipitation in the form of snow, ice and rain to Mt. Hood Saturday along with moderate winds increasing to strong and gusty winds in the afternoon (WSW gusts 40-70 mph were common at the lower NWAC wind sites!). Scoured surfaces surfaces are expected on windward slopes as well as wind effected snow well into the below treeline band. Despite heavy precipitation, only about 4 inches of new snow was recorded at the Meadows and Timberline base plots through 5 pm Saturday. The rain-line is estimated to have reached near 5500 feet Saturday mid-day before cooling ensued.

Prior to this system, 12-18 inches of storm snow fell late last week and sits on a strong firm crust layer formed earlier in the month. There are no significant layers of concern in the mid and lower snowpack.

Observations

On Saturday morning, a new rain or freezing rain crust was noted up to 6600 ft. Rime ice had covered most surfaces. Strong winds prevented observations later in the day.

Forecast for Sunday:

Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline. Avoid slopes connected to large start zones higher in the terrain where wind slab avalanches are capable of producing very large and destructive slides.

A strong cooling trend should help mitigate the storm slab likelihood, but at the same time preserve storm slab instabilities formed overnight. In non-wind affected terrain, avoid open slopes greater than 35 degrees and allow new storm slab instabilities time to stabilize.

New snow will likely bond poorly to a slick crust that formed Saturday morning near and below treeline. This would be a likely bed surface for any avalanche that steps down to deeper layers.

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.